This invention relates to a stereophonic receiver having a noise reduction control circuit.
Stereophonic broadcasting and receiving is widely available throughout the world. At the present time, stereophonic broadcasting is essentially exclusively FM (frequency modulated). However, AM (amplitude modulated) as well as television and Am systems in stereophonic are being developed. Stereophonic transmission provides two distinct signals of basically the same sounds. The two stereo signals are generated from different locations within the original sound environment, and upon reproduction create a more realistic presentation of the original sound. The signals generated and transmitted differ in phase slightly so as to permit separation at the receiver. The received signals as transmitted are processed by both left and right channels, within the receiver. The left channel transmits and reproduces essentially only the left channel signal. The right channel transmits and reproduces essentially the right channel signal in the stereo receiver.
The transmitted and processed signals include a certain level of noise which distorts the sound signal. Generally, about 20db separates the relative noise content of a stereo signal as compared to mono signal of the same strength. Means have been provided to minimize the noise content of the stereo signal. A method used is the blending of the stereo signals in the right and left channels in the receivers so as to approach somewhat a "mono" signal characteristic. A standard procedure includes passing a small value of the stereo signal of one channel to the opposite channel where it blends with the principle stereo signal of that channel. The out of phase relationship of the signals creates some cancellation of the stereo signal content. However, the random noise content is significantly reduced by the blending of the signals. Cancellation is particularly required at low level signals where the noise tends to mask and significantly distort the sound signals. At high amplitude or levels of sound, the noise content is essentially the same and thus is a significant lesser percentage of the total sound and creates relatively less perceptive distortion. Thus, depending on the receiver construction, the "stereo" and "mono" quieting approach each other and the same level at about 3000 microvolts. As the result the need for noise cancellation is correspondingly minimized.
There is a need for an improved method in high quality stereo systems of noise reduction which operates without distortion, or loss of content in the signal of the stereophonic sound received and reproduced. The present invention is particularly directed to such a noise reduction blending system which reduces the noise content with minimum deterioration in the reproduction of the stereo sound content.